In-situ Electrochemical Diagnostics for Morphological Study of CO₂ Reduction Electrodes

Electrochemical CO2 reduction (CO2R) devices have immense potential to capture the effluent CO2 in hard-to-decarbonize sectors and convert them to valuable chemicals like CO, C2H4, CH4, alcohols etc. Inside these devices, CO2R occurs inside gas diffusion electrodes (GDE). The reaction rate strongly depends on the local environment at the catalyst-electrolyte interface like ionomer binders, pH, electrolyte cations etc[1]. The transport of reactants and products to and from the catalyst-electrolyte interface depends on the pore-space structure of the GDE, which comprises mostly of micro and smaller mesopores. Ionomer binders used in these GDEs also affect the Faradaic efficiencies (FE) of the CO2R products. Moreover, experimental results indicate that the KOH fed as the anolyte in alkaline CO2 electrolyzers impact the performance of CO2R at the cathode [2]. At present, there is a lack of simple experimental methods in the literature to systematically study the local electrochemical environments present in real decices and identify critical limiting phenomena. We have developed a novel setup to study the morphological aspects of CO2 electrodes at the electrode level without the complexities due to water splitting at the anode. A stable reference electrode, an ion-exchange membrane, and the CO2R cathode are integrated together to form a membrane electrode assembly (MEA) for the purpose of precise and reproducible electrochemical experimentation. AC impedance spectroscopy (EIS) is used to measure capacitance and ion-transport resistance of the CO2R electrodes. A novel MEA setup, previously developed in this group to study oxygen mass transport resistance (MTR) in non-Platinum group metal fuel cells electrodes, is used to measure MTR of CO2 in the CO2R electrodes [3]. We vary the experimental conditions used in the CO2R experiments, like feed gas RH, KOH flow rate etc., and measure the above-mentioned properties. In addition, we also used these diagnostic methods to study the role of ionomer, specifically the ionomer-catalyst interaction in these electrodes. Experiments done by our group members indicate that a suitable electrode ink recipe (catalyst and ionomer loading, organic solvent etc.) is required to optimize the performance of CO2R inside these electrodes. Overall, these techniques can be used to understand the CO2R trends of various electrodes and to identify key design parameters for more efficient CO2 reduction electrodes. Important references: Bui et al., Engineering Catalyst–Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag; Acc. Chem. Res. 2022, 55, 484−494. Dinh et al., CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface; Science, (2018), 783-787, 360(6390). Star et a., Mass transport characterization of platinum group metal-free polymer electrolyte fuel cell electrodes using a differential cell with an integrated electrochemical sensor; Journal of Power Sources, (2020), 227655, 450.